An ovum and sperm
An ovum was a type of cell essential to Human procreation. Once fertilized by a sperm, the development of tissue could begin.
Chapter one of Gray's Anatomy described the ovum as "consist(ing) of a soft jelly-like material enclosed in a membrane". Cytoplasm was similar to this substance. Tom Paris and Harry Kim's Emergency Medical Hologram Replacement Program, a proposed replacement for The Doctor, recited the passage containing this information in 2374. (VOY: "Message in a Bottle")
The Doctor's lesson plan for Seven of Nine's instruction in interpersonal relationships, Love Amid the Stars: A Romantic Bestiary, included information on the ovum as part of "Lesson One: First Contact". While showing her a diagram of the Human procreation process, he described the image by saying, "Here we see how fortress ovum is besieged by countless little warriors (the sperm).". When Seven told him she was familiar with the biology of sex, he skipped ahead to "Lesson two: Encounter in a Public Place." (VOY: "Someone to Watch Over Me")